The only obvious difference that I see between Sampras motion and yours (aside from LeeD's comment about pointing your back towards the net) is that just after impact, your racket hitting surface faces towards you rather than towards the right side fence. To me, that means you are not pronating (forearm rolling) as much as Sampras'.

That moving the racquet from right to left will impart more side spin.

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The other thing I notice, although it's hard to tell, is at the point of impact, your racket is relatively straight as opposed to being at an angle with your forearm. That may be why you may not be getting the top spin you want. Harry

The "right to left" motion I was talking about is the racquet moving from your right side at racquet drop in pic 1 to the left side in pic 15.

Once the racquet is off to the left in pic 15, it then can move from left to right as it moves up to contact in pic 19 or 20, and continues to move from left to right after contact in the follow through in pics 21-28.

[My original point was that to get the racquet moving up from the right side at racquet drop and starting the swing from the right to the left gets the head of the racquet moving much faster in the "pronation" movement than simply bringing the racquet up from the "back scratch" position and moving it from left to right from there.]

Thanks Charlie, I understand about the right to left to right thing now. One last question: how do you control the amount of spin when you pronate through contact? For example, I can hit a flatter forehand or a spinnier forehand: how do I do the same for the serve with pronation?

OP, you have a very good serve if you are a rec player. That said, you don't think you are jumping too much forward into the court? See Sampras and Roddick in the above post where their left feet are landing.

Thanks Charlie, I understand about the right to left to right thing now. One last question: how do you control the amount of spin when you pronate through contact? For example, I can hit a flatter forehand or a spinnier forehand: how do I do the same for the serve with pronation?

On your ground stokes, topsin is created by changing your stroke pattern so you drop the racquet head lower coming out of a back swing, and finish higher.

The serve by its very nature is low to high.

So you are changing the swing path by first tossing to a different position.
That causes your upper body, and hence your arm and racquet to come through a different swing path to strike the ball.
(But you are still pronating on all the serves, and the actual swing - but not the path the upper body takes that swing through - is basically the same in all the types of serves.)

For a topspin serves, the toss is more to your left, and you strike it lower. You coil your shoulders more/lean more to maintain the same orientation you would of your shoulders to the ball placement in a "first serve", but now in a different plane.
As you come up from your leg thrust and coil your upper body will move more in a left to right path, and more straight vertically up, without as much movement into the court as on a "flat" serve.
This orientation of your upper body results in a racquet path moving almost straight up during the point of contact as in the first picture above.
(The ball is given more of a glancing blow as it strikes it at more of an angle than on a "flat serve".)

On a slice serve, the toss is more to the right and you strike it higher than on a topspin serve.
You can see the different racquet path from a flat or kick serve in the second set of pictures.
The result is the ball is struck with a sideways, not up, brushing motion as seen in the first picture above.

Darn it, I still can't do it! I can carve/supinate around the ball for spin, pronate square into it like unscrewing a lightbulb from the ceiling for power, or do a combination of both (as seen in the video), but I can't get heavy spin with heavy pronation. It's like the two seem to be mutually exclusive, but then I see Sampras and Stosur, and for them, it's clearly not. Is it the pronation that gives you the spin (if so, how?), or is it ulnar deviation like Toly says (and if so, how do you combine ulnar deviation and pronation into one motion?)? I'm probably frustrating you, Charlie, but I can assure you, I'm more frustrated than you are.

Try using a more backhand grip than your normal conti, then fully pronate (or try to). That adds the extra spin, the stronger than continental grip. Turn more sideways, or the ball goes way out to our right.

One thing, slow down during the set-up and especially the knee bend. Just relax during the toss and prep. Stay loose, smooth, and easy. Save the explosive motion for when it matters - swinging at the ball.

Darn it, I still can't do it! I can carve/supinate around the ball for spin, pronate square into it like unscrewing a lightbulb from the ceiling for power, or do a combination of both (as seen in the video), but I can't get heavy spin with heavy pronation. It's like the two seem to be mutually exclusive, but then I see Sampras and Stosur, and for them, it's clearly not. Is it the pronation that gives you the spin (if so, how?), or is it ulnar deviation like Toly says (and if so, how do you combine ulnar deviation and pronation into one motion?)? I'm probably frustrating you, Charlie, but I can assure you, I'm more frustrated than you are.

Try to analyze these pictures.

__________________
Anatoly Antipin - one of the most delicate tennis players in the world.

Darn it, I still can't do it! I can carve/supinate around the ball for spin, pronate square into it like unscrewing a lightbulb from the ceiling for power, or do a combination of both (as seen in the video), but I can't get heavy spin with heavy pronation. It's like the two seem to be mutually exclusive, but then I see Sampras and Stosur, and for them, it's clearly not. Is it the pronation that gives you the spin (if so, how?), or is it ulnar deviation like Toly says (and if so, how do you combine ulnar deviation and pronation into one motion?)? I'm probably frustrating you, Charlie, but I can assure you, I'm more frustrated than you are.

You supinate your forearm in the power position so that you can obtain maximum radial deviation, ie backscratch position. You have to pronate at the end of the upswing to square the face, then you ulnar deviate to generate spin, in conjunction with the ball position, body lean and arm angle, all of which vary slightly. Your right shoulder feels like it is rolling up and over. Most of the power comes from shoulder internal rotation amd elbow extension. The forearm goes into extreme pronation on the followthrough as the elbow flexes.